Proprioception, the perception of position and movement, plays a critical role in motor coordination. During the last funding period, the representation of movement kinematics by proprioception was shown to be complex in the human elbow. Just two kinematic variables, position and velocity, are inadequate to explain normal human proprioception. It has been determined that position is perceived as three distinct variables rather than one: static position, and two forms of dynamic position (absolute joint angle and relative angular distance). Humans can perceive independently static position, both forms of dynamic position, and velocity. This independence of perception leads to the hypothesis that static position, both forms of dynamic position, and velocity are independently represented in the discharge of proprioceptive afferents. In the proposed study, this hypothesis will be tested by comparing perception of each kinematic variable to firing patterns of human muscle spindle afferents. The analysis of firing patterns will test the hypothesis that muscle spindles represent all four kinematic variables, partly by a functional subdivision of the population, and partly by representation of multiple variables by each afferent. The subdivision is based on the presence or absence of background firing. The proposed study has three specific aims. Specific Aim 1 is to investigate how the CNS perceives static position, both types of dynamic position, and velocity. Human subjects will perform a series of perceptual-motor tasks involving wrist rotations to determine whether wrist proprioception includes the same four kinematic variables as the elbow. Specific Aim 2 is to identify features of afferent firing patterns that potentially represent each of the four kinematic variables. Responses of muscle spindle afferents will be recorded during wrist rotations identical to those used in the perceptual-motor tasks. Specific Aim 3 is to determine what representations of each kinematic variable are understood by the CNS. Tendon vibration will be used to distort systemically features of the afferent firing patterns associated with each kinematic variable. The proposed experiments involve three novel experimental techniques that were developed during the last funding period. A perceptual-motor task will be used that is unique in terms of the level of understanding of underlying proprioceptive mechanisms. Nerve recording from behaving humans (microneurography) will be carried out during an unprecedented spectrum of movement waveforms. Tendon vibration will be implemented with a unique piece of equipment in which the frequency, amplitude and force of stimulation can be precisely controlled.